A. Field of the Invention
The present invention relates to a damper mechanism, and in particular to a lockup damper and a lockup mechanism in a torque converter for mechanically transmitting a torque from an input rotary member to an output rotary member.
B. Description of the Background Art
In general, the damper mechanism transmits a torque from an input rotary member to an output rotary member, and simultaneously operates to absorb or damp a vibration transmitted from the input rotary member toward the output rotary member. A lockup mechanism disposed inside the torque converter is one example of the above damper mechanism.
The torque converter is internally provided with three kinds of vane wheels, i.e., an impeller, a turbine and a stator, and is operable to transmit a torque through a working fluid filling an internal space thereof. The impeller is fixed to a front cover coupled to the input rotary member. The working fluid flowing from the impeller to the turbine through the stator transmits a torque from the impeller to the turbine, and then is transmitted to the output rotary member coupled to the turbine.
The lockup mechanism is disposed between the turbine and the front cover for mechanically coupling the front cover and the turbine together and thereby directly transmitting the torque from the input rotary member to the output rotary member.
Usually, the lockup mechanism has a piston member which can be pressed against the front cover, a retaining plate fixed to the piston member, coil springs carried by the retaining plate and a driven member elastically coupled, in a rotating direction of the mechanism, to the piston member through the coil springs. The driven member is fixed to the turbine coupled to the output rotary member. The components of the lockup mechanism also form a lockup damper mechanism for absorbing and damping an applied vibration.
When the lockup mechanism operates, the piston member slides on or is pressed to the front cover so that the torque is transmitted from the front cover to the piston member, and then is transmitted to the turbine through the coil springs. The lockup mechanism transmits the torque, and also operates to absorb or damp the torsional or angular vibration owing to the lockup damper. The coil springs are repetitively compressed between the retaining plate fixed to the piston member and the driven member, and thereby slide on the retaining plate so that the vibration is damped. A minute torsional vibration is absorbed by repetitive elastic deformation (expansion and contraction) of the coil springs.
In the conventional lockup damper described above, outer portions, in the radial direction of the damper, of the coil springs are covered with outer bent portions of the retaining plate, i.e., outer peripheral portions which are bent.
When the lockup mechanism operates and the respective portions rotate, centrifugal forces act on the coil springs and other portions of the torque convertor so that the coil springs as well as spring seats supporting the opposite ends of the coil springs are pressed against the outer bent portions of the retaining plate. When the coil springs in this state expand and contract, the ends of the coil springs and the spring seats attached to the spring ends cause a frictional resistance with respect to the outer peripheral portions so that the damper characteristics change. In particular, the minute torsional vibration cannot be absorbed sufficiently due to presence of the frictional resistance.
A large torsional vibration often occurs during clutch engaging and disengaging operations of the lockup mechanism. In this case, the frictional resistance, if present, can effectively absorb the vibration. The damper characteristics having the above characteristics are effective in some kinds of vehicles.
The lockup mechanism provided at its radially outer portion with the coil or torsion springs can reduce an axial size of the torque converter compared with the lockup mechanism provided at its radially middle portion with the torsion springs, but cannot ensure a sufficiently large torsion angle compared with the latter. If the torsion springs are shifted from the middle position to the outer position without changing the size of the springs, an allowed maximum torsion angle between the input rotary member and the output rotary member decreases. As a result, the allowed maximum torsion angle of the lockup mechanism decreases, which reduces the property of absorbing the torsional vibration particularly in a low rotation speed range of the engine.
In order to overcome the above problems, such a structure may be employed that two or more torsion springs are arranged in series with an intermediate member(s) therebetween for increasing a total compressible size of the elastic member. This structure can provide the large torsional angle of the lockup mechanism.
However, the torsion spring having a circumferentially increased size is likely to be deformed such that a circumferentially middle portion protrudes radially outward when compressed. This tends to increase a frictional resistance between the torsion spring and a member disposed radially outside the spring. Since the lockup mechanism in the engaged state rotates together with the torque converter, a centrifugal force acts on the torsion springs. Due to this, the torsion springs tend to move radially outward and cause a frictional resistance with members arranged outside the springs. When the frictional resistance between the torsion springs and the outer members increases, the torsional vibration cannot be absorbed sufficiently.